This invention relates to a key top member for a push button switch structure suitable for use for a data input device, a switch device or the like for mobile communication equipment such as a mobile phone or a car phone, a home telephone, an electronic notebook, a measuring instrument, a vehicle mounted-switch, a remote control, a computer, a personal computer, or the like, and a method for manufacturing the same, and more particularly to a key top member for a push button switch structure which permits a display section such as a symbol, a letter, a pattern or the like printed on a sheet to exhibit enhanced design characteristics and durability and is used for a back-lighted push button switch structure as well.
In general, a push button switch structure for equipment such as mobile communication equipment like a mobile phone, an electronic notebook, a measuring instrument, a remote control or the like is so constructed that a key top member or a cover having key top elements arranged thereon is mounted on a circuit board received in a casing of the equipment, to thereby function as a switch structure for carrying an ON-OFF operation of a circuit of the equipment. Such a push button switch structure typically has a key top member incorporated therein, which is formed of thermoplastic resin into a predetermined configuration by injection molding. Recently, it has been demanded to apply a variety of designs such as a symbol, a pattern and the like to the key top by printing. This is carried out by in-mold techniques or insert molding of applying a variety of designs to a resin sheet by printing, interposedly incorporating the printed resin sheet in a mold for injection molding of a key top and then carrying our drawing of the printed resin sheet concurrently with molding of the key top by means of a pressure of resin being injected rather than techniques of directly subjecting a key top to printing.
Unfortunately, such in-mold forming or injection molding requires an expensive mold for injection molding which is constructed to have a special structure. Also, it causes molding conditions to be severe, leading to a deterioration in productivity and causes a material for the molding to be subjected to a restriction, resulting in an increase in manufacturing cost. For example, the injection molding is carried out while keeping the printed resin sheet placed in the mold, therefore, the printed resin sheet is required to exhibit enhanced heat resistance. Also, the in-mold forming or injection molding requires to provide the mold with an injection gate for each of portions of the printed resin sheet to be subjected to drawing. More specifically, it is required to provide the mold with at least one gate for every configuration of key top elements constituting the key top to be molded. Thus, formation of a plurality of key top elements by molding causes the mold to be expensive and complicated in structure. Also, it is required to inject molten resin for drawing of the printed resin sheet at a high temperature under an increased pressure while adjusting injection conditions for the gates in association with each other. This requires to precisely control temperature conditions, pressure conditions and the like for each of the resin and mold.
Further, in the in-mold forming or injection molding, a portion of the printed resin sheet to be positioned on each of the key top elements is extended by the molten resin, to thereby cause a configuration of a design previously formed on the printed resin to be deformed or shifted, leading to a deterioration in aesthetic properties of the key top finished.